1. Field of the Invention
The invention relates to an electric power steering system that assists steering operation of a driver.
2. Discussion of Background
There has been known an electric power steering system that assists steering operation of a driver by giving power of a motor to a vehicle steering mechanism. Conventionally, as the electric power steering system of this type, there is a system described in JP2006-175940 A.
An electric power steering system described in JP2006-175940 A includes a first normative model that determines a target steering torque based on a steering angle, and a second normative model that determines a target rudder angle (a target steered angle) of a steering system based on a steering torque. Driving of a motor is controlled based on these normative models (ideal models). That is, it is possible to constantly maintain the steering torque at an optimal value by a first assist component obtained by performing torque feedback control to cause an actual steering torque to follow the target steering torque. Further, it is also possible to cancel reverse input vibration from steered wheels by a second assist component obtained by performing steered-angle feedback control to cause an actual steered angle to follow the target steered angle.
In the meantime, for example, in a case where a vehicle that is running on a low μ road (a road with a low friction coefficient μ) skids when the vehicle is turning, an oversteering state occurs. In this case, a driver may operate a steering wheel in a reverse direction with respect to a turning direction of the vehicle so as to steer steered wheels in the reverse direction with respect to the turning direction of the vehicle. This operation is effective for stabilizing the vehicle, and is generally called counter steering.
On the other hand, in the electric power steering system described in JP2006-175940 A, if a steered angle of the steered wheels is changed so as to be apart from the target steered angle at the time when the driver performs counter steering, an assist torque to bring the steered angle back to the target steered angle acts on a steering mechanism. That is, an assist torque in a reverse direction with respect to a direction of the counter steering acts on the steering mechanism. Therefore, a reaction felt by the driver at the time of the counter steering becomes large, which may cause the driver to feel the sense of discomfort.